1. Field of the Invention
This invention relates to an invertor device for AC power units, and more particularly to an invertor device of the pulse width-modulation type for use in portable AC power units.
2. Prior Art
In recent years, portable AC power units have been increasing in number which are equipped with invertor devices for stabilizing the output frequency. For example, a portable power unit which generates AC power having a commercial frequency by means of an AC generator driven by an engine is known, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 59-132398, in which the engine is operated within a high speed range to cause the AC generator to generate high AC output, the output is converted into direct current, then the direct current is converted into alternating current by an invertor device, and the alternating current is outputted.
In the meanwhile, there is a demand for AC power units of this kind whose output waveform is as close to a sine wave dependent upon their application as possible. To this end, an attempt has been made to employ an invertor device of the pulse width-modulation type as the above-mentioned invertor device (Japanese Provisional Patent Publication (Kokai) No 60-82098).
According to this publication, a sine wave signal which determines the output frequency and the output voltage is compared with a triangular wave signal as a modulation carrier by a comparator, and a pulse width-modulated signal having a pulse width dependent upon the comparison result is supplied to the invertor device for switching control thereby.
To make the waveform of the output from the AC power unit using the invertor device of the pulse width modulation type closer to that of a sine wave, the frequency of the modulation carrier (the frequency of the triangular wave signal) is required to be as high as possible. However, the operating speed of the above invertor is limitted by the response time of the comparator, which is usually approximately 1 .mu.second, so that it is difficult to increase the frequency of the modulation carrier, which prevents the output waveform from becoming closer to that of a sine wave.
Further, in such AC power units as described above, generally, the output voltage is detected, and feedback control is performed based on the detected output voltage to maintain the output voltage at a predetermined value. For example, as disclosed in Japanese Provisional Patent Publication (Kokai) No. 63-167677, the output voltage is detected by means of a transformer, and the feedback control is performed based on the detected output voltage, or alternatively, an RC filter is connected to the input of an output circuit which provides the output voltage and comprises a low-pass filter, and the output voltage is detected via the RC filter so that the feedback control is performed based on the invertor output voltage detected via the RC filter.
However, according to the former conventional method of detecting the output voltage by means of the transformer, if the output voltage is offset, it is impossible to detect an offset component, and hence it is impossible to feedback-control the output voltage such that the offset component is eliminated. On the other hand, the latter method in which the invertor output voltage is detected via the RC filter requires to provide insulating transfer means, modulating transfer means, etc. to take out an accurate feedback signal from the detected voltage, which makes complicated the processing of the feedback signal to reduce or eliminate distortions in the feedback signal or a residual offset component therein.